Citalopram, a specific inhibitor of the reuptake mechanism for serotonin (5-HT), will be studied during maturation from birth to adulthood in mice. Functional studies will focus on the timing of events in the maturation of body temperature and temperature preference. Single acute doses will be used as well as repeated doses to study effects of chronic administration. Accompanying studies of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) in brain will be used to monitor the drug's action, and studies of 5-HT turnover will be used to observe effects on control of synthetic rates in the pathway. The highly specific effect of this drug will allow new information to be obtained in three areas: 1) effects of increased 5-HT in the synaptic areas outside 5-HT terminals from short-term studies; 2) longer-term effects on turnover rates in the 5-HT pathway; and 3) possible modification of maturation of 5-HT neurons from chronic administration. Information about the process of maturation will provide further insight into the function of the adult system. Citalopram represents a class of drugs which are being developed for clinical use, particularly as antidepressants. In this application it will be useful to have specific information on the drug's effect in chronic administration during brain maturation, which is completed after birth.